Assays for the purine phosphoribosyltransferase enzymes were developed to test for prokaryotic gene expression in human cells. These assays uncovered a greater complexity in the bacterial purine pathways than had been exptected. This complexity may explain the results obtained when infecting mammalian cells with the bacterial virus, lambda gpt, carrying prokaryotic genes concerned with purine metabolism. Evidence has been obtained which indicates that this virus may not contain a structural gene but instead may contain a purine control element or a posttranslational modifier. The purine assays were also used to investigate the Gilles de la Tourette syndrome. It had been suggested that self-mutilating Tourette patients have a temperature senstive purine enzyme (hypoxanthine-guanine phosphoribosyltransferase) with an unusual isoelectrofocusing pattern. We examined 11 Tourette syndrome patients, 10 relatives and 3 normal controls. Although 3 of our cases experienced self-mutilation no consistant temperature sensitivity was observed for the HGPRT enzyme. HGPRT purified by immunoprecipitation from the patients lyzed red blood cells demonstrated normal isoelectrofocusing, and SDS molecular weight patterns, using (2D) electrophoresis techniques. The use of (2D) electrophoresis and immunoprecipitation has been extended to screen for biochemical abnormalities in Huntington's chorea and other involuntary movement disorders.